The present invention relates to a fibrous heating element which can be woven or knit into fabrics, such as fabric-forming yarns and can be attached to objects by sewing. The invention also relates to a method for the production of such a heating element, and a fabric heating element made with that fibrous heating element.
As means of heating or keeping various instruments and devices warm, flexible heating wires comprising a fine metal wire have been conventionally used. Such heating wires are widely utilized in various products such as electric blankets and carpets.
While Nichrome wires are conventionally used for the heating element in products which are required to be flexible, such as the above-mentioned household products, either a flexible core having a very fine resistance wire spirally would thereon or a fabric having carbon particles bonded thereon by a resin binder has been used as the heating element.
For example, in Japanese patent publication No. 52-14449 there is disclosed a planar heating element which comprises an electroconductive cloth of a glass-fiber fabric having tinned copper wires woven therein and coated with silicone-type electric conductive paint.
However, the known planar heating elements represented by the above example are poor in pliability, and they fail to meet required characteristics with respect to bend yield resistivity and friction or abrasion resistance. In addition they lack flexibility for apparel use or medical use.
There have been several attempts to obtain fibrous heating elements which comprise a pliable yarn coated with carbonaceous particles. For example, Japanese patent application Kokai (=laying-open) publication No. 51-109321 discloses an invention, according to which a conjugate filament having a sheath component of a low melting point is subjected to heating to have the sheath component swelled and to have carbonaceous particles attached to and/or contained in the filament. This heating element has a positive temperature coefficient of resistance and does not need temperature control means. However, according to the above method of having carbonaceous particles attached to filaments, particularly where the filaments have a large diameter, it is difficult to obtain a filament containing carbonaceous particles in a sufficient amount to provide the resistance value required for a heating element. According to an example recited in that publication, the filament obtained has an electric resistivity as high as 10.sup.7 .OMEGA./cm, which is not satisfactory for a heating element.
Japanese patent publication No. 58-25086 discloses coating a fiber of a heat shrinkable polymer with an electric conductive layer and then heat-treating the fiber to obtain a product fiber having a low resistance value per unitary length. However, the invention of this publication is directed to an improvement in the electrostatic property of carpets, and the electric resistance value of the fiber obtained is only about 3.3.times.10.sup.7 .OMEGA./cm, and the products according to this invention are not useful as a heating element.
In Japanese utility model publication No. 38-1470, there is disclosed a pliable heating element which comprises a reinforcement fiber and a heating layer coated on the fiber, wherein the heating layer is an electric conductive rubber or plastic. However, the electric resistivity of the pliable heating element is only 50 to 100 .OMEGA./cm, and it is ineffective as a heating element.
In Japanese utility model publication No. 40-15750, there is disclosed an electric conductive fiber which comprises an insulator fiber and electric conductive particles adhered to the insulator fiber. However, in this method, it is difficult to adhere carbon particles uniformly, and an electric conductive fiber having the required electric resistance value is difficult to obtain.
Japanese patent publication No. 46-23357 discloses a method of producing an electric conductive fiber, which comprises adhering a paste like mixture of a polyurethane, a solvent and electric conductive particles on a synthetic filament, and forming electric conductive coating by removing the solvent. However, the electric conductive fiber is intended to be anti-electrostatic, and the electric conductive fiber is not suitable for uses as a heating element because of its low electric resistivity of less than 10.sup.9 .OMEGA./cm.
A first object of the present invention is to provide a fibrous heating element which has an electric resistance value higher than that of metals but lower than that of anti-electrostatic fibers, excellent flexibility and resistant to breaking on bending and abrasion, and which can be processed by weaving and knitting, and is usable as a sewing yarn or thread.
A second object of the invention is to provide a method for the production of such a fibrous heating element.
A third object of the invention is to provide a fabric heating element produced by forming the above fibrous heating element into a form of a pliable fabric, which can be attached to textile products such as clothes, for example, by sewing.
According to the present invention, the above objects have been attained by providing a fibrous heating element produced by coating a core fiber with at least one electric conductive layer of a polyurethane resin containing the carbonaceous particles dispersed therein.
The electric conductive layer according to the invention comprises a polyurethane resin containing carbonaceous particles dispersed therein, and gives a resistance element having a resistivity value remarkably greater than comparable values of metal resistance elements but lower than those of anti-electrostatic fibers. The heating element comprising a core fiber coated with at least one conductive layer according to the invention is pliable and resistant to breakage when bent or abraded, so that the fabric heating element made of such a fibrous heating element exhibits pliability and processability similar to those of conventional fabrics.